IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) communication, called WiFi, in the unlicensed (2.4, 3.6, 5, and 60 GHz) frequency bands. The standards and amendments provide the basis for wireless network products using the WiFi frequency bands. For example, IEEE 802.11ac is a wireless networking standard in the 802.11 family providing high-throughput WLANs on the 5 GHz band. Significant wider channel bandwidths (20 MHz, 40 MHz, 80 MHz, and 160 MHz) were proposed in the IEEE 802.11ac standard. The High Efficiency WLAN study group (HEW SG) is a study group within IEEE 802.11 working group that will consider the improvement of spectrum efficiency to enhance the system throughput in high-density scenarios of wireless devices. Because of HEW SG, TGax (an IEEE task group) was formed and tasked to work on IEEE 802.11ax standard that will become a successor to IEEE 802.11ac.
In IEEE 802.11ac, a transmitter of a BSS (basis service set) of certain bandwidth is allowed to transmit radio signals onto the shared wireless medium depending on clear channel assessment (CCA) sensing and a deferral or backoff procedure for channel access contention. For a BSS of certain bandwidth, a valid transmission sub-channel shall have bandwidth, allowable in the IEEE 802.11ac, equal to or smaller than the full bandwidth of the BSS and contains the designated primary sub-channel of the BSS. Based on the CCA sensing in the valid transmission bandwidths, the transmitter is allowed to transmit in any of the valid transmission sub-channels as long as the CCA indicates the sub-channel (or full channel) is idle. This dynamic transmission bandwidth scheme allows system bandwidth resource to be efficiently utilized.
An enhanced distributed channel access protocol (EDCA) is used in IEEE 802.11ac as a channel contention procedure for wireless devices to gain access to the shared wireless medium, e.g., to obtain a transmitting opportunity (TXOP) for transmitting radio signals onto the shared wireless medium. The simple CSMA/CA with random back-off contention scheme and low cost ad hoc deployment in unlicensed spectrum have contributed rapid adoption of WiFi systems. Typically, the EDCA TXOP is based solely on activity of the primary channel, while the transmit channel width determination is based on the secondary channel CCA during an interval (PIFS) immediately preceding the start of the TXOP. The basic assumption of EDCA is that a packet collision can occur if a device transmits signal under the channel busy condition when the received signal level is higher than CCA level.
Today, WiFi devices are over-populated. Dense deployment has led to significant issues such as interference, congestion, and low throughput. Raising CCA levels has been shown to increase spatial re-use, which leads to significant increase in the network throughput in some dense deployment scenarios. In dense deployment scenario with multiple small BSS footprints in which APs and non-AP STAs are mostly exchanging frames at the highest MCS (modulation and coding), the baseline CCA level −82 dBm leads to excessive deferral and thus lower overall throughput. For a specific link in the preceding scenario, the highest throughput is achieved approximately at an modified CCA level in which SNR for max MCS≈received signal level/(OBSS interference+noise), where OBSS interference≈CCA level. Note that if CCA level=−82 dBm, the OBSS interference is substantially below the required noise level. By increasing CCA level (OBSS interference) for all BSSs in the scenario, the operating SNR is still above the level required for max MCS. The specific link throughput does not degrade, but CCA deferral is reduced (likelihood of channel access increased) leading to increased network throughput. The network throughput increases until CCA level (OBSS interference) reaches the SNR for max MCS. Above that level, the individual link MCS degradation is to be balanced with increased likelihood of channel access (from increasing the CCA level). In general, raising CCA level can introduce more collision in the networks. It also increases device power consumption due to retries and is unfair to legacy stations since they still use the baseline CCA level.
Similar network throughput increase can be achieved in some dense deployment scenario by lowering the transmit power of all stations (STAs), which also reduces power consumption. However, when there are both legacy and IEEE 802.11ax STAs co-exist in the same environment, only reducing the transmit power of the IEEE 802.11ax STAs can lead to their performance degradation. This is because legacy STAs transmission might not deter for reduced transmit power IEEE 802.11ax STAs (when the received signal falls below the CCA of legacy STAs), but not vice versa. In order to entice IEEE 802.11ax STAs to perform TPC, it is necessary to also allow it to increase its CCA. As a result, an IEEE 802.11ax STA increases its channel access (i.e., higher CCA) but also reduces its transmit power level (lower transmit power) such that it does not cause collision.
It is desirable to have a solution 1) to increase the spatial re-use without causing collision; 2) to maintain fairness between stations in different BSSs; 3) to maintain fairness between HEW stations and legacy stations; and 4) to maintain power efficiency.